1. Field of the Invention
The present invention relates to an information recording apparatus, particularly an optical disk drive, which records data by applying energy to a recording medium to form thereon local physical changes of the medium.
2. Description of the Related Art
Optical disks currently prevailed are roughly divided into magneto-optical disks and phase change disks. In the case of magneto-optical disks, a mark of an inverted magnetic domain is formed on a recording film by heating the film, and in the case of phase change disks, a mark of an amorphous region is formed on a recording film by changing a cooling speed of the film under the control of an energy amount when it is heated. In order to increase a recording density of such optical disks, the size of a data carrying mark is reduced, or each change unit of the mark length and space length is made short to narrow a time interval between mark edge detections. In each of these methods, it is essential to form a mark at a high precision. It is very difficult, however, to stably and highly precisely form a fine mark about a half of a light spot diameter. This is because a fine mark is required to be formed on a recording film at an area having a gentle spatial temperature gradient of the film whose temperature is raised by a light spot, particularly at an area having the gentle spatial peak temperature gradient. If an effective recording sensitivity of a recording film varies, because of a change in the peak temperature at each mark caused by a change in a recording film temperature before application of recording energy or by a change in the recording energy intensity, the mark shape is deformed greatly. In the case of an optical disk of the type that the mark shape is controlled by a recording waveform, a peak temperature of a recording film is likely to fluctuate depending upon a recording pattern. A shortest approach to solving this problem is to use a light source of short wavelength laser to reduce the light spot diameter. However, wavelengths of current semiconductor laser diodes as typical laser light sources are still unsatisfactory in order to meet the requirements of increasing a recording density.
It is therefore necessary to select a recording waveform hard to pose such problems, in order to reliably form a fine mark and perform highly reliable recording/reproducing. The problems associated with a recording waveform to be solved are the following two problems. The first problem is related to suppression of thermal crosstalk to uniformly form nearby marks independently from the interval therebetween. The second problem is related to a constant heat accumulation to uniformly form consecutive marks independently from their lengths. If the thermal crosstalk suppression and constant heat accumulation can be realized, edge shifts of a reproduction signal can be suppressed so that a mark edge recording method suitable for high linear recording density can be adopted. If constant heat accumulation can be realized, reproduced crosstalk can be made constant so that the track interval can be shortened and the recording area density can be improved.
In a first conventional technique of solving the above problems disclosed in JP-A-5-298737, the recording waveform corresponding to a mark forming period is constituted of a series of pulse trains corresponding to the lengths of marks in a channel data sequence. The number of pulses and the width of pulses are controlled in accordance with the lengths of marks in the channel data sequence. The recording waveform corresponding to the mark forming period is divided into two portions, a front portion and a succeeding portion, and generally the height of each pulse is different. In a mark non-forming period of the recording waveform, a space portion is provided before an auxiliary recording pulse which is generated during this period. The mark forming period reflects the length of a mark in the channel data sequence, and is defined as shown in FIG. 4 at (c) as a period from the first pulse leading edge to the last pulse trailing edge, the pulse having an energy level sufficient for supplying energy of recording a mark, i.e., the pulse having such an energy level as a mark cannot be formed without this level. The mark non-forming period reflects the length of a space in the channel data sequence, and is defined as a period other than the mark forming period. The above definitions are applied also to the following description of this specification. The first conventional technique with the above-described structure holds the position that thermal diffusion directly from the preceding mark formed portion to the immediately succeeding mark leading edge can be compensated independently from the space length, and that the mark width and mark edge position can be controlled at high precision.
In a second conventional technique of solving the above problems disclosed in JP-A-1-078437, with reference to the length of a preceding mark non-forming period, a portion of the recording waveform corresponding to the immediately succeeding mark forming period is made variable. Specifically, as illustrated in FIG. 4 at (a), a recording energy irradiating means is provided which with reference to the lengths of preceding spaces 401 and 403, the recording waveforms corresponding to marks 402 and 404 are controlled, or more precisely, the leading edge forming positions of the marks 402 and 404 are controlled. This second conventional technique holds the position that thermal diffusion directly from the preceding mark formed portion to the immediately succeeding mark leading edge can be compensated independently from the space length, and that the mark width and mark edge position can be controlled at high precision.
Another conventional technique disclosed in JPA-5-143993 describes that if the blanking period between an immediately preceding light pulse and a current light pulse is short, heat generated by the immediately preceding light pulse influences the current light pulse and therefore this preheat effects are made to have the same effects as the long blanking period, and that the energy level and width of a light pulse supplied from a bias light irradiating unit provided immediately before the current light pulse are determined in accordance with the measured pulse width of the preceding light pulse and the measured blanking period.
Regarding the first problem, although each of the conventional techniques changes the conditions of forming a leading edge of a succeeding mark, this condition change is not satisfactory. Furthermore, each of the conventional techniques does not take into consideration the compensation for the thermal diffusion near to the leading edge forming position of an immediately succeeding mark in accordance with the level of the energy amount used for the formation of the preceding mark. Therefore, only if the preceding mark forming periods are constant, the succeeding mark can be reliably formed irrespective of the length of the mark non-forming period between two marks. However, if the immediately preceding mark forming period changes, it is difficult to form the leading edge of a succeeding mark at a target position even if the mark non-forming period between two marks is constant. Namely, the longer the preceding mark forming period, the more the heat energy used for the formation of the preceding mark diffuses near to the leading edge forming position of the immediately succeeding mark, and the nearer the leading edge comes to the trailing edge of the immediately preceding mark. The above phenomena become more conspicuous as the linear recording density is raised or the mark non-forming period between two marks becomes short.
Regarding the second problem, each of the conventional techniques is unsatisfactory with respect to the constant heat accumulation, and cannot suppress sufficiently a shift of the trailing edge from a target position depending upon the mark width, if the mark width is shortened and the linear recording density is raised. Namely, the trailing edge position and mark width fluctuate depending upon the distance from the mark leading edge. The above phenomena become conspicuous as the linear recording density is raised.
From the above reasons, therefore, each of the above conventional techniques cannot form a fine mark at a sufficiently high precision and therefore cannot realize a sufficient recording area density.